Already, that tension between growth and margins is frustrating investors, who have sent Amazon's stock down some 10 percent this year.

If Prime is at the center of Amazon's growth strategy, then content and shipping terms will need to improve, analysts said. It will have to provide a more compelling value proposition over its competitors, who now include everyone from Netflix to Wal-Mart.

As for the company's earnings, which will be revealed after the closing bell, analysts polled by Thomson Reuters expect a loss of 15 cents per share on revenue of $19.34 billion. That would be 23 percent higher than the $15.70 billion in last year's second quarter. The company had a loss of 2 cents per share in that quarter a year ago.